Although cellular phones are used primarily for making and receiving telephone calls, additional functionalities are being included in the cellular phones to allow users, for example, to take digital pictures, to listen to songs, to watch videos and/or to play games. These functionalities place a heavy demand on the cellular phone battery, which reduces the charged life of the battery. In a cellular phone, one of the most power-draining components is the power amplifier that transmits signals to the nearest cellular station. Therefore, the power-added efficiency (PAE) of a power amplifier, which is a measure of how much power is needed to achieve a given amount of amplification, is an important factor for the battery life of a cellular phone. With a higher PAE of the power amplifier, the amount of time that a user can talk on the cellular phone and/or operate other functionalities of the cellular phone can be increased.
In a power amplifier of a cellular phone, an impedance transformation network is used to reduce the impedance of the load (often 50 Ohms) to a more optimal output impedance for the amplifier so that the signal from the amplifier can properly be generated for the load. Although there are various impedance transformation networks, a conventional impedance transformation network of interest is an impedance transformation network that utilizes a set of stacked shunt ferroelectric varactors as a variable capacitor. Such an impedance transformation network is used in a power amplifier for cellular technologies that use variable transmission signal strength, such as Code Division Multiple Access (CDMA) technology, to achieve greater PAE for power levels other than the maximum power level. In contrast to semiconductor varactors, ferroelectric varactors have a higher Q factor, which makes these devices attractive for power amplifier applications.
However, the stacked shunt ferroelectric varactors of the conventional impedance transformation network are quite non-linear, and thus, the impedance transformation network requires a significant number of stacked shunt ferroelectric varactors to achieve an acceptable linearity. Unfortunately, as the number of stacked shunt ferroelectric varactors is increased, the series resistance is also increased, which degrades the total PAE of the power amplifier. Furthermore, the stacked shunt ferroelectric varactors must provide high capacitance for high power output levels, which means that the varactors must be operated at near zero DC voltage where the varactors are more non-linear. This necessitates additional stacked shunt ferroelectric varactors to achieve the desired linearity, resulting in greater series resistance.
In view of these concerns, what is needed is an impedance transformation network, power amplifier and method for efficiently transmitting an output signal in a power efficient manner.